In an electric power steering apparatus for vehicles, a steering torque generated in a steering shaft by an operation of a steering handle is detected, and then the steering auxiliary command value for assisting the steering force based on the detection result is calculated (for example, Patent Document 1).
In order to enable the calculation of the steering auxiliary command value according to the steering torque, an assist map which is information showing the correspondence relationship between the torque and the steering auxiliary command value is stored in the electric power steering apparatus. In the electric power steering apparatus, the tuning of the assist map is important in order to improve steering feel.
For example, FIG. 18 and FIG. 19 are views illustrating examples of a tuning technique of the assist map. In FIG. 18 and FIG. 19, the horizontal axis represents the steering torque (Nm) and the vertical axis represents the steering auxiliary command value (current: A).
In the example illustrated in FIG. 18, a plurality of points where the steering feel is preferable (set of a steering torque and a steering auxiliary command value) are determined. Then, the space between adjacent points is interpolated by a linear function, and the parameters of the linear functions are stored as the assist map.
Also in the example illustrated in FIG. 19, a plurality of points where the steering feel is preferable are determined. Then, a high order polynomial approximating the plurality of points is derived, and the parameters of the high order polynomial are stored as the assist map.